The present invention relates to a power control system for a semiconductor diode laser. Such a power control system is necessary to ensure that the power supplied to a diode laser is at a consistent, predetermined level both for the application to which the laser is being applied, as well as to optimize the operating life of the laser diode. More particularly, the present invention relates to a power control circuit which may be used in an optical recording, read/write, system in which the laser diode must be controlled at two separate operating power levels at different points of operation.
U.S. Pat. No. 4,166,985, issued Sept. 4, 1979, describes a system for controlling a semiconductor laser diode. The characteristics of a laser diode can change both with temperature and age as well as other possible variables of operation. The patent shows that a controlled bias current circuit, as well as circuitry to control pulses generated by the laser, are useful for prolonging life and optimizing output. The present invention is different, however, in that two different power levels of laser output must be maintained, the read power level and the write power level. Also, the characteristics of generating a higher powered write pulse power level are different when the initial condition consists of a laser read power level than the case shown in the patent, where the initial condition is a bias current and a non-laser condition.
U.S. Pat. No. 4,277,846 shows a laser power control system in which the light output of a laser is monitored by a control system which thereafter controls the laser to operate within certain limits. However, this control circuit does not concern itself with the problem of operating a laser light source between two separate laser power levels, but concerns itself with the problem of the power level between an on condition and an off condition.
U.S. Pat. No. 4,307,469 contains a low pass filter which makes the circuit slow to respond in changes in laser power output level for control purposes. In addition, this circuit deals with average power levels rather than the power level in pulses. The present invention deals with the need to control both a continuous read power laser level, as well as the power output in write pulses. In particular, the problem of dealing with the power level in write pulses is to provide a consistent pulse output characteristic even though the characteristic of the laser diode will change due to aging. The present invention relates to a need for analyzing power levels in short duration pulses, which would not be met in this patent.
U.S. Pat. No. 4,319,203 shows another system of providing a current control for a semiconductor laser diode. Again, this circuit contains low pass filters which make the circuit slow to respond to changing conditions and in particular, create a situation in which it would not respond well to changing power levels between the first and second power level at which the laser diode is operated. In addition to which, the bias current in this patent is not controlled as a function of the pulse frequency, and thus will drift away from the established value if there are no pulses at all. The circuit of the present invention must work properly to retain the predetermined write power level even if there is a zero duty cycle or short duty cycle at the write power level. That is, the present invention avoids drifting away from the predetermined values, unlike the patent.
U.S. Pat. No. 4,399,566 shows another circuit for controlling the power applied to a laser diode. Similarly, this patent deas with the mean power level of a laser diode and does not immediately operate at the predetermined power level and is slow to respond to changes and operating conditions which would affect changed requirements to meet the predetermined power levels.
U.S. Pat. No. 4,122,409 shows a laser power control system in which laser power may be controlled at different levels. While it mentions that digital control signals may be used (col. 5, line 35), it does not show the use of independent digital stepping switches to maintain the same effective power output level of multiple levels or a normalizing converter so that all power levels are converted to a standard level for monitoring purposes.
The present invention operates a laser source at two different predetermined power levels, one of which may be a relatively continuous duty cycle and the other of which may be a relatively short or zero duty cycle, and can also quickly make necessary adjustments to control both duty cycle power levels within predetermined limits.